Migrate from scrypt to sha256. (#8969)

This reduces the server time to compute the hash from 40ms to 5µs,
which should remove this as a noticable chunk of CPU time in production.

(An attacker who has access to our database will now need only 10^54
years of CPU time instead of 10^58 to brute force a token).

Release Notes:

- Improved sign in latency by 40ms.

crates/collab/Cargo.toml

@@ -23,6 +23,7 @@ aws-config = { version = "1.1.5" }
 aws-sdk-s3 = { version = "1.15.0" }
 axum = { version = "0.6", features = ["json", "headers", "ws"] }
 axum-extra = { version = "0.4", features = ["erased-json"] }
+base64.workspace = true
 chrono.workspace = true
 clock.workspace = true
 clickhouse.workspace = true
@@ -48,6 +49,7 @@ serde_derive.workspace = true
 serde_json.workspace = true
 sha2.workspace = true
 sqlx = { version = "0.7", features = ["runtime-tokio-rustls", "postgres", "json", "time", "uuid", "any"] }
+subtle.workspace = true
 rustc-demangle.workspace = true
 telemetry_events.workspace = true
 text.workspace = true